The VHD image is Microsoft's VirtualPC image of IE7. I am trying to clone it in order to get a new UUID so that I can also use the IE8 image in VirtualBox. Right now both images share the same UUID so I am only able to use one of them.

Should be fixed in 2.2.4. However, the .vhd backend is currently not able to omit writing empty blocks with the result that the target .vhd will be blown to the full final size. In the case of the MS IE7 .vhd this is 16GB. For the moment I suggest you to copy the .vhd and assign a new UUID to the target. VBoxManage internalcommands sethduuid works for .vhd images since VirtualBox 2.2.4.

I just tested it with 2.2.4 and it appears to have been fixed. I tested it with the IE7 VHD from the site linked to in the original post. But as noted by frank in the comments, the file size is huge so it is recommended to use the VBoxManage internalcommands sethduuid command.

The is the better solution IMHO. Just copy the VHD and change the UUID. Repeat for as many copies as you need.